The interaction between electromagnetic radiation, i.e. x-ray radiation as well, and a medium is described, inter alia, with the aid of the complex refractive index. Here, the real part and imaginary part of the refractive index in each case depend on the material composition of the medium to which the complex refractive index is assigned. While the imaginary part reproduces the phenomenon of absorption of electromagnetic radiation in a medium, the real part describes the phenomenon of refraction, which is mainly known from the field of geometric optics.
In principle, both interaction phenomena can be used for an imaging method, for example for medical diagnostics. However, devices for metrological acquisition of the material-dependent absorption are predominantly used at this moment in time, wherein, in so-called x-ray radiography, an object to be examined is irradiated by x-ray radiation and the transmitted intensity downstream of the object to be examined is recorded. On the basis of this measurement, it is possible to make a projection image which reproduces a two-dimensional distribution of the absorption properties of the object. Finally, in x-ray tomography, a multiplicity of projection images are recorded, on the basis of which a three-dimensional data record is then calculated, said three-dimensional data record reflecting the spatial distribution of the absorption coefficients. From this, it is then possible to deduce the spatial distribution of the material composition of the object.
These days, devices for acquiring the phase shift, i.e. the refraction of x-ray radiation, caused by an object to be examined in order to establish the material composition of objects are still less widespread. The corresponding methods, these being referred to as phase contrast radiography and phase contrast tomography, however, are currently subject to intensive research and appropriate devices are increasingly being developed.
Here, the metrological acquisition of the phase shift is generally brought about indirectly by measuring an intensity and an interference condition, wherein the metrological design employed for this is typically based on a Talbot-Lau interferometer. Accordingly, it comprises a number of optical and, in particular, x-ray optical gratings disposed between an x-ray radiation source and an x-ray detector. Such a design enables the implementation of an interferometric measurement method, as emerges from e.g. “X-ray phase imaging with a grating interferometer”, T. Weitkamp et al., Optics Express, volume 13, number 16, Aug. 8, 2005.
Further machines for phase-contrast imaging are known, inter alia, from the European patent application EP 1 447 046 A1 and the German patent applications 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 291.4, 10 2006 015 356.1 and 10 2006 015 355.3, and various embodiments of x-ray optical gratings are described in e.g. DE 10 2006 037 281 A1.
As mentioned above, the starting point of all known measurement devices is formed by the Talbot-Lau interferometer, in which an x-ray radiation source, a coherence grating G0, a phase grating G1, an analysis grating G2 and an x-ray detector made up of a plurality of pixels are arranged along an optical axis. Here, the coherence grating G0 serves to ensure a sufficient spatial coherence of the x-ray radiation source.
Accordingly, it is possible to dispense with the coherence grating G0 in the case of an x-ray radiation source which, to a good approximation, can be considered to be a point source. An interference pattern is generated with the aid of the phase grating or else a diffraction grating G1, which typically has a uniformly striped structure, wherein the period of this interference pattern is typically significantly smaller than the size of the pixels of the x-ray detector, and therefore direct acquisition of the interference pattern with the x-ray detector is not possible.
Rather, the analysis grating or the absorption grating G2, with the aid of which the interference pattern can, as it were, be sampled by a spatial-periodic masking of x-ray radiation, is arranged upstream of the x-ray detector, and so said interference pattern can be displayed with the aid of the pixels of the x-ray detector. To this end, the analysis grating G2 is displaced in a plane perpendicular to the optical axis and also perpendicular to the stripes of the structure in the case of a uniformly striped structure of the phase grating G1 and intensity measurements are undertaken with the aid of the x-ray detector for different displacement positions.
Alternatively, a stationary arrangement is provided for the analysis grating G2 and the interference pattern is sampled by displacing the coherence grating G0 or the diffraction grating G1.
This basic design can be used for an imaging method, i.e. for phase-contrast imaging, wherein an object to be examined or a patient is positioned e.g. between the coherence grating G0 and/or the x-ray radiation source and the phase grating G1, or between the phase grating G1 and the analysis grating G2 in the vicinity of the phase grating G1. The object to be examined then causes a spatially dependent phase shift of the x-ray radiation passing through the object to be examined, as a result of which changes are generated in the interference pattern which are acquired metrologically by way of the x-ray detector.
In order to establish the phase shift caused by the object to be examined, there a displacement of, for example, the phase grating G1 across the optical axis in discrete steps, wherein the intensity at the x-ray detector is measured in each case. The measurement values acquired thus are then for example compared to sinusoidal curves associated with different phase shifts. In this manner it is then possible to deduce the phase shift caused by the object to be examined.